WASHINGTON — President Biden announced on Thursday a hard-earned bipartisan agreement on a pared-down infrastructure plan that would make a start on his top legislative priority and validate his efforts to reach across the political aisle. But he openly acknowledged that Democrats will likely have to tackle much of the rest on their own.

The bill’s cost of $973 billion over five years, or $1.2 trillion over eight years, is a scaled-back but still significant piece of Biden’s broader proposals.

It includes more than a half-trillion dollars in new spending and could open the door to the president’s more sweeping $4 trillion proposals for child care and what the White House calls human infrastructure later on.

“When we can find common ground, working across party lines, that is what I will seek to do,” said Biden, who deemed the agreement “a true bipartisan effort, breaking the ice that too often has kept us frozen in place.”

President Biden, with a bipartisan group of senators that includes Maine Sen. Susan Collins, speaks Thursday outside the White House in Washington about the infrastructure deal. Evan Vucci/Associated Press

The president stressed that “neither side got everything they wanted in this deal; that’s what it means to compromise,” and said that other White House priorities would be taken on separately in a congressional budget process known as reconciliation, which allows for majority passage without the need for Republican votes.

He insisted that the two items would be done “in tandem” and that he would not sign the bipartisan deal without the other, bigger piece. House Speaker Nancy Pelosi and progressive members of Congress declared they would hold to the same approach.

“There ain’t going to be a bipartisan bill without a reconciliation bill,” Pelosi said.

Claiming a major victory five months into his presidency, Biden said, “This reminds me of the days when we used to get an awful lot done up in the United States Congress.” Biden, a former Delaware senator, said that as he put his hand on the shoulder of a stoic-looking Republican Sen. Rob Portman of Ohio as the president made a surprise appearance with a bipartisan group of senators to announce the deal outside the White House.

And Sen. Susan Collins, R-Maine, said, “It sends an important message to the world as well that America can function, can get things done.”

But the next steps are not likely to be nearly so smooth.

Senate Republican leader Mitch McConnell complained that Biden was “caving” to Pelosi and Senate Majority Leader Chuck Schumer’s plan to “hold the bipartisan agreement hostage” for the president’s bigger package of what he called “wasteful” spending.

“That’s not the way to show you’re serious about getting a bipartisan outcome,” McConnell said.

And there is plenty of skepticism on Biden’s own left flank. Democratic Sen. Richard Blumenthal of Connecticut said the bipartisan agreement is “way too small — paltry, pathetic. I need a clear, ironclad assurance that there will be a really adequate robust package” that will follow.

Thursday’s deal was struck by the bipartisan group led by Portman and Democrat Kyrsten Sinema of Arizona, including some of the more independent lawmakers in the Senate, some known for bucking their parties.

“You know there are many who say bipartisanship is dead in Washington,” Sinema said. “We can use bipartisanship to solve these challenges.”

The proposal includes both new and existing spending on long-running programs and highlights the struggle lawmakers faced in coming up with ways to pay for what have typically been popular ideas.

The investments include $109 billion on roads and highways and $15 billion on electric vehicle infrastructure and transit systems as part of $312 billion in transportation spending. There’s $65 billion toward broadband and expenditures on drinking water systems and $47 billion in resiliency efforts to tackle climate change.

Rather than Biden’s proposed corporate tax hike that Republicans oppose or the gas tax increase that the president rejected, funds will be tapped from a range of sources — without a full tally yet, according to a White House document.

Money will come from $125 billion in COVID-19 relief funds approved in 2020 but not yet spent, as well as untapped unemployment insurance funds that Democrats have been hesitant to poach. Other revenue is expected by going harder after tax cheats by beefing up Internal Revenue Service enforcement that Portman said could yield $100 billion.

The rest is a hodgepodge of asset sales and accounting tools, including funds coming from 5G telecommunication spectrum lease sales, strategic petroleum reserve and an expectation that the sweeping investment will generate economic growth — what the White House calls the “macroeconomic impact of infrastructure investment.”

The senators from both parties stressed that the deal will create jobs for the economy and rebuild the nation’s standing on the global stage, a belief that clearly transcended the partisan interests and created a framework for the deal.

“We’re going to keep working together — we’re not finished,” Republican Sen. Mitt Romney of Utah said. “But America works, the Senate works.”

Democratic Sen. Jon Tester of Montana said it will show the world “we’re not just, you know, a hot mess here.”

For Biden, the deal was a welcome result. Though for far less than he originally sought, Biden had bet his political capital that he could work with Republicans toward major legislation.

Moreover, Biden and his aides believed that they needed a bipartisan deal on infrastructure to create a permission structure for more moderate Democrats — including Sinema and Joe Manchin of West Virginia — to then be willing to go for a party-line vote for the rest of the president’s agenda.

The announcement leaves unclear the fate of Biden’s promises of massive investment to slow climate change, which Biden this spring called “the existential crisis of our times.”

Biden’s presidential campaign had helped win progressive backing with pledges of major spending on electric vehicles, charging stations, and research and funding for overhauling the U.S. economy to run on less oil, gas and coal. The administration is expected to push for some of that in future legislation.

Sen. Bill Cassidy, R-La, stressed that there are billions of dollars for resiliency against extreme weather and the impact of climate change and deemed Thursday’s deal a “beginning investment.”

Biden has sought $1.7 trillion in his American Jobs Plan and the $1.8 trillion American Families Plan for child care centers, family tax breaks and other investments that Republicans reject as far outside the scope of “infrastructure.”

The broad reconciliation bill would likely include tax increases on the wealthy, those earning more than $400,000 a year, and hike the corporate rate from 21% to 28%, so a tension still exists over funding for some Republicans and business groups.

It’s still a long haul to a bill signing at the White House. The Senate expects to consider the bipartisan package in July, but Biden’s bigger proposal is not expected to see final votes until fall.